The invention is directed to a gold bath for the electrolytic (galvanic) deposition of high gloss and ductile gold alloy coatings based on potassium cyanoaurate (III) and an acid. In the electro industry today the predominant part of gold coatings are deposited from weakly acid electrolytes which operate in the pH range of 3.5-5. They contain the gold in the form of potassium dicyanoaurate (I), KA.mu.(CN).sub.2 and as buffer, salts of weak inorganic or organic acids such as phosphates, citrates, or phosphonates. By the addition of metal salts, above all salts of nickel, cobalt, indium, the properties of the deposited layer are strongly influenced. There are obtained from these baths under conditions in which 0.2-0.5% Ni or Co are codeposited, coatings which are glossy, have a hardness of 150-180 HV, and a good resistance to mechanical abrasion. Beginning with a thickness of about 5.mu. these coatings are practically pore-free. They are distinguished by good electrical conductivity and low contact resistance.
The coatings from these baths of course also have considerable disadvantages. Their ductility is low. With light mechanical deformation they form cracks. They contain a relatively large (about 1%) amount of non-metallic impurities which probably lead to functional disturbances in contacts for electronic connectors. Probably also because of these impurities, the contact resistance does not remain constant by heating and damages the function of the electrical apparatus.
The content of the alloying metal in the coating is strongly dependent on the current density and the pH so that the working conditions must be held within narrow limits in order to maintain coatings with constant quality. The average current density is limited to about 1 A/dm.sup.2. The bath temperature must not be higher than 40.degree. C. and in most baths the pH must be held between 3.5 and 4. At a pH of 4, the coatings are frequently only milky-glossy or already matte. Operation at pH values below 3.5 cannot be used since the KAu(CN).sub.2 decomposes with separation of gold cyanide, AuCN. In spite of this there were attempts to use such baths at pH below 3 (e.g. German AS No. 1262723 and Atwater U.S. Pat. No. 2,978,390), the results, however, were not satisfactory.
It was also tried to operate with baths in the acid range which contain the complex of trivalent gold with potassium cyanide, the potassium tetracyanoaurate (III), KAu(CN).sub.4.
In Freedman U.S. Pat. No. 3,598,706, there is described a process for the production of the tetracyanogold (III) acid, and a bath based thereon. In Fletcher U.S. Pat. No. 4,168,214 there is described a bath for plating a gold strike on stainless steel whereby the tetracyanogold (III) complex is formed by reaction of gold (III) chloride with potassium cyanide in the bath and there is established a pH between 0.1 and 1.5.
These known baths can also contain additional alloying metals. A pure alloy bath for the deposition of gold-tin-coatings based on KAu(CN).sub.4 in hydrochloric acid medium is claimed in German OS No. 2658003. (U.S. Pat. No. 4,013,523).
Baths in which the gold is present in trivalent form in contrast to the baths based on monovalent gold salts have the disadvantage that because of the difference in valence at equal current densities and times only one-third the amount of gold is deposited. This disadvantage can only be balanced by the use of correspondingly higher current densities. However, this is not possible with previously known baths since then the deposited layers are matte and rough so that thick coatings are not economically producible. A further substantial disadvantage is the high chloride content in these baths through which an implement development of chlorine occurs at the anode. If the baths additionally contain hydrochloric acid then there occurs besides corrosion problems on the plant.
Therefore it was the problem of the present invention to find a gold bath for the electrolytic (galvanic) deposition of gold alloy coatings (i.e. gold alloy platings) based on potassium cyanoaurate (III) and an acid which are both high gloss and ductile even at high current densities and in which no chlorine development occurs.